1. Field of the Invention
The present invention relates to a light-emitting element using a phosphorescent compound. Further, the present invention relates to a light-emitting device using the light-emitting element. Furthermore, the present invention relates to an electronic device using the light-emitting device.
2. Description of the Related Art
In recent years, a light-emitting element using a light-emitting organic compound or inorganic compound as a light-emitting substance has been actively developed. In particular, a light-emitting element called an EL element has a simple structure in which a light-emitting layer containing a light-emitting substance is provided between electrodes. Therefore, the light-emitting element has attracted attention as a next-generation flat panel display element because of its characteristics such as a thin shape, lightweight, high response speed, and direct current driving at low voltage. In addition, a display using such a light-emitting element has a feature that it is excellent in contrast and image quality, and has a wide viewing angle. Further, since these light-emitting elements are plane light sources, it is considered that these light-emitting elements are applied as light sources such as a backlight of a liquid crystal display and lighting.
In a case of using a light-emitting organic compound as a light-emitting substance, the emission mechanism of a light-emitting element is a carrier injection type. Namely, by application of a voltage with a light-emitting layer interposed between electrodes, carriers (holes and electrons) injected from the electrodes are recombined and a light-emitting substance is made to be in an exited state. Light is emitted when the excited state returns to the ground state. As the type of the excited state, a singlet excited state (S*) and a triplet excited state (T*) are possible. The statistical generation ratio thereof in a light-emitting element is considered to be S*:T*=1:3.
In general, the ground state of a light-emitting organic compound is a singlet excited state. Therefore, light emission from a singlet excited state (S*) is referred to as fluorescence because of electron transition between the same multiplicities. On the other hand, light emission from a triplet excited state (T*) is referred to as phosphorescence because of electron transition between different multiplicities. Here, in a compound emitting fluorescence (hereinafter referred to as a fluorescent compound), in general, phosphorescence is not observed at room temperature, and only fluorescence is observed. Therefore, in a light-emitting element using a fluorescent compound, the theoretical limit of internal quantum efficiency (the ratio of generated photons to injected carriers) is considered to be 25% based on S*:T*=1:3.
On the other hand, when a compound emitting phosphorescence (hereinafter referred to as a phosphorescent compound) is used, internal quantum efficiency can be theoretically 75 to 100%. In other words, luminous efficiency can be three to four times as high as that of a fluorescent compound. From these reasons, in order to achieve a light-emitting element with high efficiency, a light-emitting element using a phosphorescent compound has been proposed (for example, see Non Patent Document 1 and Patent Document 1).
In addition, in recent years, by combination of a phosphorescent compound and a light-emitting material different from the phosphorescent compound, an attempt has been made to obtain a white light-emitting element with high efficiency (for example, see Patent Document 2).
[Non Patent Document 1]
Tetsuo TSUTSUI, and eight others, Japanese Journal of Applied Physics, vol. 38, L1502-L1504 (1999)
[Patent Document 1]
Japanese Published Patent Application No.2003-229275
[Patent Document 2]
Japanese Published Patent Application No.2004-311420